Ultrasound creates a variety of non-thermal effects on biological tissues ranging from necrotic damage to delicate reversible effects like permeability enhancement and excitable tissue stimulation. The present invention, in some embodiments thereof, relates to cell manipulation and, more particularly, but not exclusively, to devices and methods of manipulating cells by generating pressure wave(s in proximity thereto. In a recent study by Krasovitski et al. (2011) a novel mechanism of ultrasound induced intra membrane cavitation is suggested as the reason for ultrasound induced bioeffects in cells, tissue and organ. This mechanism denoted also as Sonophore or Bilayer Sonophore (BLS) suggests that US preferably induces bubble formation in the intra-membrane space between the two lipid leaflets.
The presence of a gas microbubble near the BLS acts as a pressure amplitude amplifier under ultrasound. It is evident today that the ability to introduce microbubbles in the human body is becoming crucial. When subjected to ultrasound, synthetic microbubbles can be used for a variety of therapeutic and diagnostic purposes. Microbubbles can be generated in the body by high intensity ultrasound or introduced intravenously as encapsulated bubbles also known as ultrasound contrast agents (UCA). In-vivo generated microbubbles may improve and widen numerous diagnostic and therapeutic applications in which UCA are currently used. Some of these applications are reviewed by E. Kimmel, Cavitation bioeffects, Critical Reviews in Biomedical Engineering 34 (2006) 105-62, which is incorporated herein by reference. One may also consider ultrasonically induced drug delivery which is based on releasing medication from broken UCAs at a specified location as described in E. Stride, N. Saffari. On the destruction of microbubble ultrasound contrast agents. Ultrasound Med. Biol. 29 (2003) 563-73, which is incorporated herein by reference to and increasing permeability of blood vessel walls for facilitated transport, which is incorporated herein by reference. Also, microbubbles are essential in ultrasonically induced targeted hyperthermia, and diagnostic imaging of specific cells, as well as for increasing membrane permeability of cells for, for instance, gene transfection, see for example R. G. Holt, R. A. Roy. Measurements of bubble-enhanced heating from focused, MHz-frequency ultrasound in a tissue-mimicking material. Ultrasound Med. Biol. 27 (2001) 1399-412, H. Ashush, L. A. Rozenszajn, M. Blass, M. Barda-Saad, D. Azimov, J. Radnay, D. Zipori, U. Rosenschein. Apoptosis induction of human myeloid leukemic cells by ultrasound exposure. Cancer Res. 60 (2000) 1014-20, and R. J. Price, S. Kaul. Contrast ultrasound targeted drug and gene delivery: an update on a new therapeutic modality. J. Cardiovasc. Pharmacol. Ther. 7 (2002) 171-80, which are incorporated herein by reference.
High Intensity Focused Ultrasound (HIFU) sources provide enough power density to initiate bubble generation and growth in-vivo. An HIFU focal pressure of 4.5 MPa produces detectable bubbles in-vivo, see C. H. Farny, T. Wu, G. Holt, T. W. Murray, R. A. Roy. Nucleating cavitation from laser-illuminated nano-particles. Acoust. Res. Let. Online 6 (2005), which is incorporated herein by reference and referred to herein as Farny (2005). For example, S. D. Sokka, R. King, K. Hynynen. MRI-guided gas bubble enhanced ultrasound heating in vivo rabbit thigh. Phys. Med. Biol. 48 (2003) 223-241, which is incorporated herein by reference, suggests the use of very high intensity ultrasound (about 7 kW/cm2 at the focal point) of brief duration (0.5 s) to generate nucleation sites. These are then evolved into microbubbles by application of much lower intensity (about 0.2 kW/cm2) insonification.
An alternative method for generating nucleation sites by exposing light-absorbing gold nanoparticles to laser was suggested by Farny (2005). Gold spherical nanoparticles as well as carbon nano tubes can serve as light-absorbing elements in-vivo because they are biocompatible, conductive and have certain geometrical characteristics that allow them to effectively transform light energy into heat (see Jain et al., see P. K. Jain, I. H. El-Sayed, M. A. El-Sayed. Au nanoparticles target cancer. Nano Today 2 (2007) 18-29, which is incorporated herein by reference. Farny (2005) demonstrated that nanobubbles of about 150 nm in diameter can be obtained by exposing gold nanoparticles embedded in gel to a 532 nm (peak absorption of near-spherical nanoparticles) laser pulse phase-synchronized with an ultrasound burst of 10 cycles, lasting about 10 μs at a frequency of 1.1 MHz. The acoustic pressure amplitude used was 0.9 MPa—namely 20 W/cm2 in a propagating wave. Farny (2005) identified the threshold at which a nanobubble, once generated by a laser pulse heating an individual nanoparticle, evolves into a microbubble. The threshold occurs at acoustic pressures near 1 MPa, for laser energy densities of about 5 mJ/cm2.
V. P. Zharov, K. E. Mercer, E. N. Galitovskaya, M. S. Smeltzer. Photothermalnanotherapeutics and nano diagnostics for selective killing of bacteria targeted with gold nanoparticles. Biophys. J. BioFast, Oct. 20, 2005, doi:10.1529/biophysj.105.061895, which is incorporated herein by reference, demonstrated that exposing bacteria-attached clusters of gold nanoparticles to 532 nm laser pulse-generated nuclei that evolve into microbubbles at laser energy densities above 100 mJ/cm2. Regarding the use of microbubbles for tissue heating, it has been indicated that microbubble oscillations can enhance local ultrasound energy deposition by two orders of magnitude, see S. Fujishiro, M. Mitsumori, Y. Nishimura, Y. Okuno, Y. Nagata, M. Hiraoka, T. Sano, T. Marume, and N. Takayama. Increased heating efficiency of hyperthermia using an ultrasound contrast agent: A phantom study. Int. J. Hyperthermia 14 (1998) 495-502, which is incorporated herein by reference.